Capillary for bonding copper wires between a semiconductor circuit chip and a corresponding terminal connector of a semiconductor device

ABSTRACT

A capillary for electrical bonding between a semiconductor chip and corresponding pins of a semiconductor device in which the chip is accommodated, comprising a body whose terminal portion is substantially frustum-shaped, the body having a diametrical through hole which allows the passage of a copper wire for electrical bonding between the chip and the semiconductor device; the portion of the body that is adjacent to a lower end of the through hole is flared, with a flaring diameter and a flaring angle which allow to form a substantially flat annular peripheral region on a copper ball when the copper ball placed at a lower end of the copper wire is deformed by the action of the capillary, the formation of the substantially flat annular peripheral region being independent of the position of the copper wire within the through hole of the body of the capillary.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a capillary for bonding copper wires between a semiconductor circuit chip and a corresponding terminal connector of a semiconductor device.

[0002] It known that it is necessary to provide electrical bonds between a semiconductor chip and corresponding pins or leads on a metallic leadframe of a semiconductor device.

[0003] The bonding pins are formed monolithically with the leadframe and allow the semiconductor circuit to be mounted and connected to a pre-printed electronic board.

[0004] Preset contact pads are arranged in the peripheral region of the semiconductor chip and must be bonded to corresponding pins or leads of the semiconductor device. Fine wires are used for this purpose between the pads and the inward ends of the corresponding pins or leads.

[0005] The electrical bonds are currently provided by means of gold and aluminum wires owing to their reliability and yield.

[0006] However, the possibility to use copper wires instead of gold and aluminum wires has recently been verified. The physical properties of copper make it particularly advantageous with respect to gold and aluminum wires. These properties can be summarized as low electrical resistance, phase stability and reliability when the copper wire is arranged on the silicon pads of the semiconductor chip, low cost and higher melting current than wires of the same diameter made of different materials.

[0007] However, the use of copper wires is hindered by two main drawbacks:

[0008] the activation problem related to the formation of loose balls. The ball is a copper sphere which is provided in order to mutually bond the copper wire and the silicon pad onto which the copper wire is to be bonded. The copper sphere is obtained by melting the copper wire, for example by applying a suitable voltage between the wire and an electrode, with consequent electrical discharge. In these conditions, the copper is subject to oxidation, and this entails the generation of a loose irregular ball and the deterioration of the soldering process

[0009] The second drawback is the fact that the copper wire is very hard, and this can lead to cracking of the silicon pad of the chip.

[0010] The first drawback related to the formation of the loose ball or sphere has been solved by using forming gas.

[0011] The cracking problem has instead been eliminated by means of a correct choice of the parameters for bonding the copper wire to the semiconductor chip pad.

[0012] However, the presence of an irregularly shaped copper ball is a drawback which the current art has not yet solved. The irregular shape of the copper ball which is formed at the end of the copper wire and therefore allows to bond said wire to the silicon pad of the semiconductor chip, is mainly due to the geometric shape of the capillary in which the copper wire is passed and is used to form the copper ball and most of all to bond it to the silicon pad of the semiconductor chip.

[0013] The optimum configuration to be obtained for said copper ball is the one in which the diametrical axis of the ball coincides with the axis of the copper wire bonded thereto.

[0014] Situations in which the copper ball is substantially tangent to the outer surface of the copper wire to which it is bonded are generally not ideal for bonding, for example ultrasonic bonding, of the copper ball to the surface of the semiconductor chip (silicon pad).

[0015]FIG. 1 is a sectional view of the end portion of a conventional capillary in which a copper wire is inserted with a copper ball at its lower end. The capillary is designated by the reference numeral 1, the copper wire is designated by the reference numeral 2 and the ball is designated by the reference numeral 3.

[0016] In this case, the copper wire 2 is perfectly centered within the hole 4 of the capillary 1, and the result of bonding the ball to the surface of a pad 5 of a semiconductor chip is shown in FIG. 2, which clearly shows that the capillary 1 applies a modeling action to the ball 3, which assumes a shape such as the one shown in detail in FIG. 2a, with a substantially flat annular peripheral region 6.

[0017]FIG. 3 instead illustrates a situation in which the capillary 1 is used to solder the copper wire 2 but the wire is not centered within the hole 4 of the capillary 1 and therefore the copper ball 3 is not shaped as in FIG. 2a, i.e. symmetrically, but as shown in FIG. 3a, which clearly illustrates how the substantially flat annular peripheral region 6 in this case is not formed symmetrically with respect to a diametrical axis of the copper ball 3.

[0018] Substantially, a deformed ball 3, as shown in FIG. 3a, would not be considered acceptable for the purpose of bonding a lead to the silicon pad of a semiconductor chip.

SUMMARY OF THE INVENTION

[0019] The aim of the present invention is to provide a capillary for the electrical bonding of a copper wire between a semiconductor chip and the corresponding pins of a semiconductor device which accommodates said chip, wherein the deformation of the copper ball is such as to produce an annular peripheral region which is substantially flat and substantially symmetrical with respect to a diametrical axis of the copper ball.

[0020] Within this aim, an object of the present invention is to provide a capillary in which the shape of the end that is adapted to make contact with the copper ball is such as to achieve, on said copper ball, an annular peripheral region which is substantially flat and symmetrical with respect to a diametrical axis of the copper ball.

[0021] Another object of the present invention is to provide a capillary which offers adequate characteristics of yield in the process for bonding the copper wire to the semiconductor chip.

[0022] Another object of the present invention is to provide a capillary which is highly reliable, relatively simple to manufacture and at competitive costs.

[0023] This aim and these and other objects which will become better apparent hereinafter are achieved by a capillary for electrical bonding between a semiconductor chip and corresponding pins of a semiconductor device in which said chip is accommodated, comprising a body whose terminal portion is substantially frustum-shaped, said body having a diametrical through hole which allows the passage of a copper wire for electrical bonding between said chip and said semiconductor device, wherein the portion of said body that is adjacent to the lower end of said through hole is flared, with a flaring diameter and a flaring angle which allow to form a substantially flat annular peripheral region on a copper ball when said copper ball placed at the lower end of said copper wire is deformed by the action of said capillary, the formation of said substantially flat annular peripheral region being independent of the position of said copper wire within said through hole of said body of the capillary.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Further characteristics and advantages of the present invention will become better apparent from the following detailed description of a preferred but not exclusive embodiment of the capillary according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein:

[0025]FIG. 1 is a sectional view of a conventional capillary;

[0026]FIG. 2 is a sectional view of the capillary of FIG. 1, with the bonding ball shown in its deformed configuration;

[0027]FIG. 2a is a detail view of the configuration of the ball for mutually bonding the copper wire and the silicon pad, already shown in FIG. 2;

[0028]FIG. 3 is a sectional view of the capillary when the copper wire contained therein is not axially centered;

[0029]FIG. 3a is a sectional view of the configuration assumed by the deformed copper ball when said copper wire is not perfectly centered within the hole of the capillary;

[0030]FIG. 4 is a sectional view, taken along a transverse plane. of the capillary according to the present invention;

[0031]FIG. 4a is a sectional view, taken along a transverse plane, of the capillary according to the invention, showing the formation of a copper ball;

[0032]FIG. 5 is a sectional view, taken along a transverse plane, of the capillary according to the invention during the deformation of the copper ball, with the copper wire arranged so that it coincides with the diametrical axis of the capillary;

[0033]FIG. 5a is a sectional view, taken along a transverse plane, of the configuration assumed by the copper ball when using the capillary according to the present invention;

[0034]FIG. 6 is a sectional view, taken along a transverse plane, of the capillary according to the present invention, illustrated in the condition in which the copper wire accommodated within the capillary is not centered in the passage hole; and

[0035]FIG. 6a is a sectional view, taken along a transverse plane, of the configuration assumed by the deformed copper ball when using a capillary as shown in FIG. 6.

DESCRIPTION OF THE REFERRED EMBODIMENTS

[0036] With reference to the figures, and particularly to FIGS. 4 to 6 a (the remaining figures, which relate to the prior art, are not discussed further herein, since they have already been described), in which identical reference numerals designate elements which are identical both in FIGS. 4 to 6 a and in the preceding figures, the capillary, generally designated by the reference numeral 100, comprises a substantially frustum-shaped body portion 11 which has a diametrical channel 4 for the passage of a copper wire 2 meant to be bonded to the silicon pad of a semiconductor chip. The copper ball 3 that forms between the pad of the semiconductor chip and the copper wire 2 must substantially be shaped, once it has been subjected to deformation by means of the capillary 1, so as to have at least one annular peripheral region 6 which lies substantially symmetrically with respect to a diametrical axis of the copper ball 3.

[0037] The particular shape of the tip of the frustum-shaped portion 1 of the capillary according to the invention is such as to apply to the ball 3 a compression which is applied substantially symmetrically with respect to a diametrical axis of the ball 3 regardless of whether the copper wire 2 is perfectly centered within the through cavity 4 of the capillary 1 or is arranged as shown in FIG. 4, i.e., substantially adjacent to one of the internal walls of the frustum-shaped body 1.

[0038] The particularity of the invention therefore resides in that it determines specific relations between the internal surfaces of the portion of frustum-shaped body 1 of the capillary according to the invention.

[0039] For this purpose, the lower portion of the frustum-shaped body 1 is shaped so as to have a flared region which blends with the through hole 4 in is which the copper wire 2 is accommodated.

[0040] The flared region is formed, in terms of flaring angle and diameter, so as to form on the ball 3, when said ball is deformed, an annular peripheral region 6 which is substantially symmetrical around the axis of the ball 3.

[0041] Therefore, using the reference numeral 11 to designate the flaring angle of the portion of the frustum-shaped body 1, the reference numeral 12 to designate the diameter of the flared region, the reference numeral 13 to designate the diameter of the through hole 4 (of the lower portion of the through hole 4) and the reference numeral 15 to designate the diameter of the copper wire, the relation that must be found between these values must be such as to produce, regardless of the position of the copper wire 2 within the passage hole 4, an annular peripheral region 6 which is substantially symmetrical with respect to the diametrical axis of the copper ball 3.

[0042] Experimental values, obtained for a copper wire with a diameter of 2 mils, are given hereafter.

[0043] For such a copper wire 2, it has been found experimentally that the optimum diameter of the hole 4 must be 55±1.5 microns; the flaring angle 11 must be 120°±5°; and the diameter of the flared region 12 must be equal to 82±2 microns.

[0044] These values of course are referred to a particular diameter of the copper wire 2, i.e., a diameter of 2 mils. For different diameters of the copper wire 2, the above-cited values will change. However, the basic concept of the invention is to define a flared region of the frustum-shaped portion of the body 1 of the capillary, said flared region constituting the lower portion of the through hole 4, such as to allow to determine an annular peripheral region 6 which is substantially symmetrical with respect to the diametrical axis of the copper ball 3.

[0045] In practice it has been observed that the capillary according to the present invention fully achieves the intended aim and objects, since it allows to determine a substantially flat annular peripheral region on the deformed copper ball, so as to -render the bonding between the copper wire and the silicon pad, by means of said ball, acceptable from an electrical viewpoint and in terms of strength of the bond.

[0046] The capillary thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept; all the details may further be replaced with other technically equivalent elements.

[0047] In practice, the materials employed, so long as they are compatible with the specific use, as well as the dimensions, may be any according to requirements and to the state of the art.

[0048] The disclosures in Italian Patent Application No. M12000A000791 from which this application claims priority are incorporated herein by reference. 

What is claimed is:
 1. A capillary for electrical bonding between a semiconductor chip and corresponding pins of a semiconductor device in which said chip is accommodated, comprising a body whose terminal portion is substantially frustum-shaped, said body having a diametrical through hole which allows the passage of a copper wire for electrical bonding between said chip and said semiconductor device, wherein the portion of said body that is adjacent to a lower end of said through hole is flared, with a flaring diameter and a flaring angle which allow to form a substantially flat annular peripheral region on a copper ball when said copper ball placed at a lower end of said copper wire is deformed by the action of said capillary, the formation of said substantially flat annular peripheral region being independent of the position of said copper wire within said through hole of said body of the capillary.
 2. The capillary according to claim 1 , wherein the diameter of said copper wire is 2 mils.
 3. The capillary according to claim 1 , wherein the diameter of said passage hole, at the lower portion of said passage hole, directly adjacent to said flared region, is in the range between 53.5 and 55 microns.
 4. The capillary according to claim 1 , wherein said flaring angle is in the range between 115 and 125 microns.
 5. The capillary according to claim 1 , wherein the flaring diameter of said flared region is in the range between 80 and 84 microns.
 6. The capillary according to claim 3 , wherein the diameter of said passage hole is 55 microns.
 7. The capillary according to claim 4 , wherein said flaring angle is 120°.
 8. The capillary according to claim 5 , wherein said flaring diameter is 82 microns. 